This invention relates generally to digital to analogue converters and more particular, though not exclusively, to such converters which are applicable to television or radio tuners in which the frequencies of resonant circuits are controlled by means of direct voltages applied to varicap diodes. The magnitude of the direct voltage is controlled by digital code signals applied to a digital to analogue converter.
One prior art type of digital to analogue converter comprises a register adapted for storing an applied digital signal, an oscillator connected to a counter and a comparator for providing an output signal upon coincidence between the values held in the storage register and the counter. This type of converter further comprises a bistable multivibrator circuit which is set by the coincidence signal and is reset upon the "full count" of the counter. The bistable circuit controls a switch that alternately couples one of two different potential sources to a smoothing circuit; and the direct voltage output from the smoothing circuit is representative of the applied digital signal.
In such a converter, the frequency of ripple on the output is dependent on the signal frequency from the oscillator and of the value of the maximum count of the counter. The time constant of the smoothing circuit must be such as to effectively remove the ripple from its output signal; however, large time constants have the disadvantage of reducing the response speed of the converter. For example, some configurations could require several seconds to attain a new value after the applied digital signal is changed.